1. Field of the Invention
This invention relates to an image reading device of an image forming apparatus. In more detail, this invention relates to an image reading device, an image reading method and an image forming apparatus each using a solid-state image sensor.
2. Description of Related Art
In recent years, a copying machine has been drastically improved to be faster and to obtain higher resolution. Thereby, an electronic circuit within the copying machine is required to be even faster. In particular, when image information converted into electric signals is to be read from a solid-state image sensor which senses image information of light, in view of two requests which are to increase the number of data points in conjunction with obtaining higher resolution and to be faster, it is required to be yet even faster. Therefore, a basic clocking signal at the time of reading image information is made faster.
However, according to the above-mentioned earlier art, when image information is to be read from a solid-state image sensor, there is a possibility of reading wrong image information. In other words, since image information in a solid-state image sensor is outputted as electric signals only during a signal period on an output timing chart, if a sampling position is not included in the signal period, sampling of image information is not accurately performed.
In particular, in conjunction with the requirement of making it faster, in a solid-state image sensor which is represented by CCD, a basic clock is made to have high frequency. Thus, the signal period generated according to the basic clock is made shorter. In addition, although a sampling position is set out in consideration of signal delay according to artwork wiring on a printed board, there is a difficulty of setting out a sampling position in consideration of variation of electric characteristics of each electric device, especially a semiconductor device.
Based on these problems, it is extremely important how to achieve the sampling of output electric signals from a solid-state image sensor within a signal period, in order to obtain accurate image information.